A Delay Tolerant Network (DTN) having a function of temporarily storing transfer data without immediately transferring to a next hop is a network system which enables end-to-end delivery of transfer data with high reliability even in a condition that connectivity between nodes is unstable and low-quality, such as a wireless ad hoc network and a satellite network.
In a DTN, transfer data is divided into unit sizes called bundles and a process of transferring to a destination is executed on each bundle. The greatest characteristic of a DTN is, when a route to the destination of a bundle to transfer is unknown, storing the bundle and waiting until a next hop to transfer the bundle is found without discarding the bundle. By performing such an operation, even when an end-to-end route from a source node to a destination node does not exist at a time, it is possible to first transfer a bundle to a halfway node and then restart transfer of the bundle at a moment that a route to the destination node is made. As a result, there is an effect that the rate of arrival at a destination node increases.
Therefore, a different method from IP routing, which is finding an end-to-end route from a source node to a destination node, is used as a route control method in a DTN. One of the typical methods for routing in a DTN is a routing method called PROPHET, which is based on the delivery predictability for a destination node.
In the PROPHET, routing based on the delivery predictability for a destination node is performed. To be specific, when one node A neighbors another node B, the node A updates delivery predictability Pi for the node B in accordance with the following equation:Pi=P(i−1)+(1−P(i−1)×Pinit  Equation 1.
In Equation 1, P(i−1) is the delivery predictability for the node B of the node A before update. Further, Pinit is an initialization constant and satisfies 0<Pinit<1.
Further, in a case where the node A and the node B remain not neighboring each other, the node A regularly updates the delivery predictability Pi by a method called aging and gradually reduces the delivery predictability in accordance with the following equation:Pi=P(i−1)×γk  Equation 2.
In Equation 2, γ is a reduction parameter and satisfies 0<γ<1. Moreover, k is the number of times that a unit time passes after the node A and the node B last neighbor each other.
When a node holds a bundle whose destination is a certain node, the node exchanges the delivery predictabilities Pi for the destination node with neighboring nodes, and forwards the bundle to a neighboring node having the highest delivery predictability. Moreover, in a case where the delivery predictabilities for the destination node of the neighboring nodes are smaller than the delivery predictability for the destination node of the node, the node retains the bundle without forwarding to any of the neighboring nodes.
Thus, in the PROPHET, routing by comparison in magnitude of the delivery predictabilities for a destination node which are calculated based on a neighboring history is performed.
Further, Patent Document 1 discloses a method of not only calculating the delivery predictability for a destination node on the basis of a neighboring history but also calculating the delivery predictability on the basis of a history of end-to-end routes to the destination node from the past to the present registered in a routing table within a node. Thus, in the PROPHET, a method extended to use as the delivery predictability not only a value calculated on the basis of a neighboring history alone but also a value calculated on the basis of another parameter as disclosed by Patent Document 1.
Patent Document 1: International Publication No. WO2009/078427
As mentioned before, a DTN enables end-to-end delivery of transfer data with high reliability even in a condition that connectivity between nodes is unstable and low-quality. However, transfer data does not always arrive at a destination node from a source node quickly, and it sometimes takes a long time to arrive. Then, it varies depending on destination nodes whether transfer data quickly arrives or it takes a considerable time to arrive. This is because the transfer data delivery predictability for a destination node from a certain node varies depending on destination nodes. It is one of the important information for users using DTN communication whether or not there is a probability that transfer data quickly arrives at a destination node. However, the transfer data delivery predictability for a destination node of each node is merely used for route control and even existence thereof is not recognized by the users. Therefore, it is difficult for the user of a source node to select a destination node by using the delivery predictability.